The serial transmission of data electronically in digital form as enables computer to computer communication is known and is in wide spread use. In that transmission, information is encoded in digital form, typically through use of the binary code consisting of a series of "ones", or highs, and "zeros", or lows, with each group of a predetermined number of consecutive bits in the serial transmission, such as 5, representing a symbol and a group of such symbols representing a "frame" or message. At the receiving end, the coded information is decoded and processed into a form useable by other electronic apparatus for processing, and/or control or display. As those skilled in the art recognize, the media used for data transmission can be of many types, typically copper wires, and in more modern systems fiber optic cables. With fiber optic cables the encoded information is converted into modulation of light; turning the light on and off at high rates so as to represent the digital information.
This latter type of medium has been recognized as the preferred form of transmission line for data transmission purposes in the local area networks, LAN, in which a large number of stations are effectively "tied" together; one station can send digital information to other stations in the loop or network, and, likewise receive information addressed to the respective station by any other station in that loop.
Professional users of network computers are familiar with the local area network as incorporated within the modern business or office for allowing communications between computers. Those networks, however, have other more critical application in military networks and, particularly, in space satellites. As those skilled in the art appreciate, a fiber optic local area data network replaces heavy and bulky wire harnesses and cabling systems. By allowing many stations to use the same transmission line and permit individual addressing of a particular station along the loop, the need for a cabling system wired between each station and every other station so as to allow direct communication, is avoided, as was recognized many years ago with the Digital Equipment Company PDP-11 System. In spacecraft applications weight is at a premium. Hence, the elimination of heavy and bulky transmission line harnesses, common to present spacecraft, is a desireable end.
The fiber optic transmission medium permits greater band width, hence, greater data transmission rates, than ordinary copper wire transmission lines. Hence, with the recent introduction and application of fiber optic systems higher data transmission rates are being achieved with which to allow more efficient and productive use of the transmission medium. Greater amounts of data may be sent and/or received in shorter periods of time. In a local area network, thus, each computer or station in the network can complete its transmission or reception more quickly, allowing the other stations in the loop to do so as well. Further, with higher data transmission speeds, accordingly, a greater number of stations may be incorporated within any given network.
As those skilled in the art recognize, a given electronic device, such as a computer, consists of many different sections that are electrically wired together. With high speed data transmission fiber optic loops it is conceivable that the individual sections of such electronic device may be connected together over a single fiber optic cable in a local area network, eliminating the copper wiring between circuits. To assure that data transmission equipment of one manufacturer is compatible with, that is may "speak to", data transmission equipment of a different manufacturer, the manufacturer's adhere to industry accepted standards in the design and operation of their equipment. For this purpose, the American National Standards for industry has established a specification for a fiber-distributed data interface, FDDI, ANSI X3T9 Series 3.1.39, which specifies the protocol and tolerances for LAN digital signal communications in the fiber optic media. That standard specifies a non-return-to-zero, NRZI, indication. The protocol requires a modulator demodulator circuit that processes data into a serial stream for transmission over a media and successfully recovers the data at the destination at rates of 100 million information bits per second and an acquisition capability within twelve symbol periods or less.
In such a data transmission system there is no separate synchronizing circuit, as would signal the receiving station separate via a separate transmission path that a symbol is to start or that message is being started and sent. The receiving station must, on its own, determine from the serial data stream presented on the network that there is data on the network being transmitted, must derive a clock signal with which to enable the determination of a start of symbol operation in the data and must decode the data so as to ascertain whether the selected station is the addressee and, if so, decode the message. To do so, the cited standards require that the receiving equipment "acquire" the incoming signal within twelve symbol periods or less for un-encoded data rates of at least 100 million bits per second. Digital modulator/demodulator circuits for performing these functions at those rates are commercially available. However, those circuits all rely upon an analog phased-lock loop recovery method. While phased-lock loop type oscillator circuits have received wide application in digital communications systems, and serve an almost indispensable part of present day systems, they have limitations.
Analog phased-locked loops are subject to degradation over long periods of time, such as, for example, the ten to fifteen year period in which space satellites on a long mission explores the cosmos. Phase locked loop oscillators are also subject to degradation through exposure to both naturally occurring and militarily induced radiation, such as occurs in parts of the stellar system and/or which might be released as energy by an atomic blast. Whereas failure of the phased-locked loop oscillator in business and office local area networks, though inconvenient, is easily repaired by telephoning the technician and giving him immediate access to the system parts. However, repair to a satellite communications system that fails two years into a stellar journey is not practical. An object of the invention, therefore, is to provide a digital transmission system and, more particularly, an modulator/demodulator for such a system that is of greater long term reliability than other systems employing a phased-locked loop oscillator.
With a modulator/demodulator of greater reliability suitable for space application, a necessary fall-out of that application is that the same circuits may be used in business and industrial application to enhance the reliability in those applications as well and minimize the need for the repair technician.
To that same end, the initial cost of an improved modulator demodulator, constructed according to the disclosed specification, must be cost competitive initially, with those prior systems containing a phase-locked loop oscillator. To that end an entirely digital circuit, as may be implemented on a single chip of semiconductor material, by large scale integration, LSI, techniques, offers a ready low cost solution. Once the initial costs of chip design are amortized, the continued production of additional copies of chips results in a lower per unit manufacturing cost. Accordingly, a further object of the invention is to provide a design for an modulator/demodulator that can be implemented on a single semiconductor chip as the preferred form.
As has long been known, Gallium Arsenide based semiconductor devices are capable of operating at greater speeds than semiconductor chips fabricated using Silicon technology. While the silicon technology is wide-spread, in part because of lower manufacturing costs and acceptable performance, the Gallium Arsenide technology, which has not received as wide use, is, at present, more expensive. The Gallium Arsenide technology may be said to have been limited in application to the very high speed devices, where the silicon technology is unsatisfactory in performance. At digital data transmission rates of 125 million cycles per second and consequent clocking rates of 500 MHz the Gallium Arsenide is preferred. Accordingly, an additional object of the invention is to provide a design for a high speed digital modulator/demodulator that may be implemented on a Gallium Arsenide semiconductor chip.
Silicon semiconductor technology need not be neglected. Although the high speeds desired as an object are perceived as requiring implementation in Gallium Arsenide materials, it is also possible to provide an modulator/demodulator circuit design that can be operated at lower speeds and be implemented in a silicon semiconductor chip. Accordingly, a still additional object of the invention is to provide a design for an modulator decoder that is entirely digital in operation that may be implemented in either of the two semiconductor technologies. By providing for operation at lower clock speeds, the circuit as implemented on a silicon-type chip should perform satisfactorily.
Modulator/demodulator circuits convert a serial stream of data into a parallel data stream, such as a five bit parallel code, which is output to succeeding circuits associated with the receiving station. The demodulator must, thus, interface with other equipment. An additional object of the invention, therefore, is to provide an improved modulator/demodulator for use in a media access control structure suitable for the high speed FDDI Token Ring Network. In that application the output signals must be predictable and reliable. An ancillary object of the invention is to provide a modulator circuit that is capable of outputting a data stream at a relatively constant rate.